Author Gavin Extence's first novel tells the story of a teenager whose adolescent awakening is more death than sex. Photograph: Gary Calton for the Observer

A teenager drives off the ferry at Dover with an urn full of human ashes on the passenger seat and a bag of marijuana in the glove compartment. Stopped at customs, he turns up Handel's Messiah on the stereo to try to avert an epileptic seizure and is taken away by the police. This is the set-up for Gavin Extence's first novel, The Universe versus Alex Woods, and the rest of the book describes, in Alex's voice, how it came about.

At 10, Alex, only son of a spiritualist shopkeeper who conceived him with a stranger somewhere near Stonehenge, is hit by a meteorite that hurtles through the roof of his home in Somerset and leaves him with a brain injury. Alex feels chosen, rather than damaged, by his fate. Among a lot of interesting information about astrophysics and neurology, which is one of the many strengths of Extence's book, is the fact that there is only one well-documented real-life case of a person being injured by a meteor: Mrs Annie Hodges of Sylacauga, Alabama, in 1954.

What Alex's personality was like before the meteor hit we don't know. Five years later, when he decides the time has come to take the iron-nickel fragment off his bookshelf and give it to the Natural History Museum, he reflects that "without the meteorite, I would have been an entirely different person. I'd have a different brain – different connections, different function." Alex is transfigured by his close encounter with outer space.

Forced to stay off school until his fits can be controlled by a mixture of meditation and medication, he reads Tolkien, conducts a charming correspondence with a meteor expert and helps in his mother's shop. Rejoining the local secondary, he is bullied, but unharmed, by thugs who one day chase him into a neighbour's garden shed.

The shed belongs to Mr Peterson, an American widower who smokes pot, loves Kurt Vonnegut and fought in Vietnam, and Alex's friendship with Peterson fills the remaining three-quarters of the book. They start a reading group, for which Alex's notice in the local library reads: "Ever wondered why we're here? Where we're going? What the point is? Concerned about the state of the universe in general?? THE SECULAR CHURCH OF KURT VONNEGUT: a book club." But these philosophical soirees (with flapjacks by Mrs Griffith) turn out to be only a brief respite from real‑life dealings with mortality, for it soon transpires that The Universe versus Alex Woods is a book about assisted dying.

The novel has many funny lines, with most of the comedy spun out of the contrast between Alex's precocity and his innocence. Lighting the candles for his mother's tarot readings, he says: "I probably added to the atmosphere, like some strange, mute goblin that would emerge now and then from the gloom to tend to the various flickering fires." But there are moments when you wonder if the novel is too winsome for its own good. Childless Mr Peterson fills the gap marked "father" in Alex's life rather too neatly, while neurologist Dr Enderby and physicist Dr Weir are like two good fairies dispensing pearls of advice on demand. "Not all scars are bad, Alex. Some are worth hanging on to, if you know what I mean," says Dr Weir at one point. Sometimes Alex is a smart alec: the chapter sending up his academy school and peer group reads like the revenge of the nerd.

But the novel won me over. Extence tells a great story that owes much to Vonnegut, but also something to Mark Haddon's The Curious Incident of the Dog in the Night-Time and John Irving's A Prayer for Owen Meany. It's hard not to see an echo of Harry Potter, too, in the boy-hero with a scar on his head. The final section is humane and touching, and Extence deserves credit for the clever and timely idea of fictionalising a trip to the Swiss death clinic.

Alex Woods is a secular hero, for whom adolescent awakening is more death than sex. For Alex, growing up means facing the laws of physics. At Cern, on his final journey with Mr Peterson, he thinks about "how old the universe was, and how old it would become before it suffered its final heat death – when all the stars had gone out and the black holes had evaporated and all the nucleons decayed, and nothing could exist but the elementary particles, drifting through the infinite darkness of space". Extence's hugely likable first novel is a fairytale for rationalists.